$ \left(\dfrac{81}{4}\right)^{\frac{1}{2}}$
Solution: Figure out what goes in the blank: $\Big(? \Big)^{2}=\dfrac{81}{4}$ Figure out what goes in the blank: $\Big({\dfrac{9}{2}}\Big)^{2}=\dfrac{81}{4}$ So $\left(\dfrac{81}{4}\right)^{\frac{1}{2}}=\dfrac{9}{2}$